Mars Formed Fast, Stayed Small

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Astronomers don't know how it happened, but Mars appears to have
sat out the game of planetary billiards that marked the solar
system's early days, surviving as a quickly formed, diminutive
and relatively unaltered embryo planet, a new study shows.

The finding offers an explanation to the long-standing mystery of
why Mars, the fourth planet from the sun, is so small, lead
researcher Nicolas Dauphas, with the University of Chicago, tells
Discovery News.

"There was this problem of explaining the small size of Mars,"
Dauphas said. "There were ideas floating around in the scientific
community, but the question was what do you measure to tell
whether these ideas are a fairy tale or whether it corresponds to
what happened."

Dauphas and colleagues decided to attack the problem by figuring
out how fast it took Mars to form, a study that entailed
painstaking analysis of radioactive materials in 20 Mars
meteorites and comparative studies of 30 chondrite meteorites,
which are believed to be leftover remains from the solar system's
earliest days.

With the ratios of radioactive tracer elements in hand, the
scientists then turned to computer simulations to figure out how
fast Mars formed. They found the planet reached half its present
size in 1.8 million years -- or less.

In contrast, Earth, which is close to twice the size of Mars,
took 50 to 100 million years to form.

The key difference appears to be access to new material. Earth
bulked up on planetoids and other bodies that crashed into its
surface, while Mars starved.

Computer models suggest Earth and Venus may have deflected
objects away from Mars, or objects may have been scattered
outward before they could be incorporated into Mars, said
University of Miami researcher Ali Pourmand.

"It appears that Mars was a lucky survivor and escaped collision
with other similar bodies," Pourmand wrote in an email to
Discovery News.

Mars' rapid growth means heat from the decay of radioactive
aluminum 26, another element present in the early universe, would
have had time to temporarily turn Mars into an ocean of magma,
Dauphas added.

"It's an important difference if a planet forms as a solid ball
or if it's molten. What you see on the surface can be influenced
by the large-scale geology of the planet, including whether the
planet has a magma ocean. That's a very important outcome of our
work," he said.

Whether the planet's rapid growth impacted its chance to host
life is not known.

"A shorter accretion timescale could potentially give life the
chance to evolve earlier on Mars. But I must emphasize this is
pure speculation," wrote Pourmand. "Every planet has a different
evolutionary history."